There are many instances in which it is both desirable and necessary to move a normally stationary, but movable, component of equipment from one position to another quickly in response to a predetermined increase in ambient temperature. For example, it is common to provide a burner assembly operable to apply heat to an object and to control the flow of combustion air or fuel to the burner by means of a movable damper or valve. It is desirable to provide an actuating mechanism which responds automatically and responds to a predetermined rise in ambient temperature for moving such damper or valve automatically from an open position to a closed position so as to terminate the delivery of combustion air to the burner. Various proposals heretofore have been made for the purposes referred to above. One example is disclosed in the U.S. Pat. No. 4,346,554. Apparatus constructed in accordance with the invention performs the same function as that disclosed in such patent, but is an improvement over the patented construction.